1. Technical Field
The invention relates to image sensor systems. More particularly, the invention relates to an image sensor architecture and associated method for facilitating image multiple sampling using a time-indexed approach to achieve a wide dynamic range.
2. Description of the Prior Art
Digital photography is one of the most exciting technologies to have emerged during the twentieth century. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Digital cameras, for example, are on the cutting edge of digital photography. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make the digital cameras one of the hottest new category of consumer electronics products.
Digital cameras, however, do not work in the same way as traditional film cameras do. In fact, they are more closely related to computer scanners, copiers, or fax machines. Most digital cameras use an image sensor or photosensitive device, such as charged-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) to sense a scene. The photosensitive device reacts to light reflected from the scene and can translate the strength of that reaction into a numeric equivalent. By passing light through red, green, and blue filters, for example, the reaction can be gauged for each separate color spectrum. When the readings are combined and evaluated via software, the camera can determine the specific color of each element of the picture. Because the image is actually a collection of numeric data, it can easily be downloaded into a computer and manipulated for more artistic effects.
Nevertheless, there are many cases in which digital cameras simply can not be used because of the limited resolution of the image sensors in today's digital cameras. Film-based photographs have immeasurably higher resolution than digital cameras. While traditional film-based technology typically has a resolution of tens millions of pixels, the image sensors in the digital cameras that could be produced at a price that is acceptable to consumers is slightly more than a million pixels today.
Dynamic range is another critical figure of merit for image sensors used in digital cameras. The dynamic range of an image sensor is often not wide enough to capture scenes with both highlights and dark shadows. This is especially the case for CMOS sensors which, in general, have lower dynamic range than CCDs.
Previously suggested solutions for widening the dynamic range of these devices can be divided into three categories:                Compressing the response curve;        Multiple sampling; and        Control over integration time.        
The response curve is compressed by using a sensor that has a logarithmic response.
There are two ways of doing this:                The first approach is to use a CMOS sensor that operates in an instantaneous current read out mode. In this mode, the photocurrent generated by a photodetector is fed into a device that has a logarithmic response, for example a diode connected MOS transistor, to compress the sensor transfer curve. Although this scheme can achieve very wide dynamic range, the resulting image quality is generally poor due to a low signal-to-noise ratio (SNR).        The second approach to compress the response curve uses a technique referred to as well capacity adjusting. Here, the dynamic range is enhanced by increasing well capacity one or more times during exposure time. During integration well capacity is monotonically increased to its maximum value. The excess photo-generated charge is drained via an overflow gate. This scheme, however, suffers from large fixed pattern noise and degradation in the SNR.        
In multiple sampling, a scene is imaged several times at different exposure times and the data are combined to construct a high dynamic range image. For this approach to work at reasonable capture times, the read out process must be performed at speeds much higher than normal active pixel sensor (APS) speeds. The multiple sampling scheme effectively achieves a wide dynamic range. In reality, much data may need to be read out, which can be particularly burdensome for many types of image sensors.
Controlling integration time is the third method that has some promising aspects in comparison with others. In essence, the exposure time of each pixel is individually adjusted so that they do not get saturated at the end of each integration period. There are many ways of achieving this.
One way is to place a set-reset flip-flop and an AND gate at each pixel to control the integration start time to achieve local exposure control. However, this approach suffers the following limitations:                Each pixel is large due to the inclusion of the flip-flop and the AND gate.        A large ‘timestamp’ memory is needed to store the exposure time of all pixels. The exposure time of each pixel can be determined by trying out various exposure times. When capturing a moving scene, the exposure times change so the ‘timestamp’ memory must be updated, which not only is burdensome but also causes image lag.        Moreover, in addition to the column and row decoders used for pixel read out, another column and row decoders are needed to control the flip-flops.        
A second way is known for an individual pixel reset (IPR) to achieve local exposure control, namely a second reset transistor is added to the standard three-transistor APS design so that the integration start time of each pixel can be controlled externally. The second way keeps the pixel size small but requires a large external memory to store the exposure time for all of the pixels, and further requires memory refreshing and additional column and row decoders. Moreover, multiple reset pulses might need to be applied to each pixel throughout the reset period. The time control for resetting pulses could be quite complicated.
There is therefore a great need for a wide dynamic range image sensor that overcomes some of the above shortcomings and, in particular, outputs image data having a wide dynamic range. Further, the sensor should not require an external timestamp memory and control logic to update the exposure times.